Add missing use statement

Author: Seldaek

src/Monolog/Handler/BufferHandler.php

@@ -13,6 +13,7 @@
 
 use Monolog\Logger;
 use Monolog\ResettableInterface;
+use Monolog\Formatter\FormatterInterface;
 
 /**
  * Buffers all records until closing the handler and then pass them as batch.